Presentation on theme: "The Chromosomal Basis of Inheritance Chapter 15. The Chromosomal Theory of Inheritance Mendelian genes have specific loci (positions) along chromosomes."— Presentation transcript:
The Chromosomal Basis of Inheritance Chapter 15
The Chromosomal Theory of Inheritance Mendelian genes have specific loci (positions) along chromosomes. It is the chromsomes that undergo segregation and independent assortment.
Fig P Generation Yellow-round seeds (YYRR) Y F 1 Generation Y R R R Y r r r y y y Meiosis Fertilization Gametes Green-wrinkled seeds ( yyrr) All F 1 plants produce yellow-round seeds ( YyRr ) R R Y Y r r y y Meiosis R R Y Y r r y y Metaphase I Y Y RR r r y y Anaphase I r r y Y Metaphase II R Y R y y y y R R Y Y r r r r y Y Y R R yR Yr yr YR 1/41/4 1/41/4 1/41/4 1/41/4 F 2 Generation Gametes An F 1 F 1 cross-fertilization 9 : 3 : 1 LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. LAW OF SEGREGATION The two alleles for each gene separate during gamete formation
Thomas Hunt Morgan Drosophila melanogaster Wild Type- the phenotype for a character most commonly observed in natural populations. Mutant Phenotypes- traits that alternatives to the wild type. A student at UK
EXPERIMENT P Generation F1F1 All offspring had red eyes Fig. 15-4b RESULTS Generation F2F2
The Chromosomal Basis of Sex Determination SRY- Sex determining Region of Y -The presence of this gene on the Y chromosome codes for the development of testes. In the absence of this gene, the gonads develop into ovaries
Sex Linked Genes A gene located on either sex chromosome is called a sex-linked gene In humans, sex-linked usually refers to a gene on the larger X chromosome For a recessive sex-linked trait to be expressed -A female needs two copies of the allele -A male needs only one copy of the allele Sex-linked recessive disorders are much more common in males than in females
Fig XNXNXNXN XnYXnY XNXnXNXn XNYXNY XNXnXNXn XnYXnY N= normal color vision
Morgans Experiments Linked Genes- genes located on the same chromosome that tend to be inherited together in genetic crosses.
What type of ratios would you expect to see in the testcross offspring if the genes were located on different chromosomes? What if they were located on the same chromosome and parental alleles are always inherited together?
Morgans Experiments Linked Genes- genes located on the same chromosome that tend to be inherited together in genetic crosses. Genetic Recombination- the production of offspring with combinations of traits that differ from those found in either parent.
YyRr Gametes from green- wrinkled homozygous recessive parent ( yyrr ) Gametes from yellow-round heterozygous parent (YyRr) Parental- type offspring Recombinant offspring yr yyrrYyrr yyRr YRyr Yr yR Parental Types- offspring that inherit a phenotype that matches one of the parental phenotypes. Recombinant Types or Recombinants- offspring that have new combinations of phenotypes.
Recombination Crossing Over- accounts for the recombination of linked genes. Genetic Map- an ordered list of the genetic loci along a particular chromosome. Linkage Map- a genetic map based on recombination frequencies. Displays order but not precise location. –Distances are expressed in map units- equivalent to 1% recombination frequency. (Centimorgans)
Exit Slip In humans, pseudohypertrophic muscular dystrophy is a condition in which the muscles gradually waste away, ending in death in the early teens. This type of muscular dystrophy is dependent upon a sex-linked recessive gene that occurs only in boys and has never been reported in girls. Hypothesize why it does not appear in girls? Why does the sex-linked gene for pseudohypertrophic muscular dystrophy not become eliminated from the human race since all boys showing the trait die before reaching maturity?
Alteration of Chromosome Number Nondisjunction- pairs of homologous chromosomes do not separate normally during meiosis –As a result, one gamete receives two of the same type of chromosome and the other gamete doesnt receive a copy. Aneuploidy- a chromosomal aberration in which one or more chromosomes are present in extra copies or deficient in number Polyploidy- a chromosomal alteration in which the organism possesses more than two complete chromosome sets.
Fig Meiosis I Nondisjunction (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II Meiosis II Nondisjunction Gametes Number of chromosomes n + 1 n – 1 nn
Alteration of Chromosome Structure Deletion- removes a chromosomal segment Duplication- repeats a segment Inversion- reverses a segment within a chromosome Translocation- moves a segment from one chromosome to another
Exceptions to the Standard Chromosome Theory Genomic Imprinting- a variation in phenotype depending on whether an allele is inherited from the male or female parent. Extranuclear Genes- genes outside of the nucleus. Some are located in organelles in the cytoplasm.